Since high-temperature parts, such as rotor blades and stator blades of industrial gas turbines, and flame tubes, tail pipes, and split rings of combustors, etc., are used in high-temperature environments, they are generally provided with a thermal barrier coating on the surface.
FIG. 11 is a sectional view of a conventional thermal barrier coating.
The conventional thermal barrier coating film is arranged by laminating a metal binding layer 12 of MCrAlY alloy on a base material 11 of a rotor blade or the like and then further laminating a ZrO2 (zirconia)-based ceramic layer 13, for example, a layer of a partially stabilized ZrO2 which is partially stabilized by the addition of Y2O3 at a proportion of 6 to 8 wt % (hereinafter referred to as “YSZ”) on the metal binding layer 12 as a topcoat. Herein, the M in MCrAlY represents a solitary element or a combination of two or more elements selected from Ni, Co, Fe and the like.
However, for recent gas turbines the turbine entrance temperature has been increasing and thus higher thermal barrier properties are being demanded of topcoats. Also, thermal stress due to the thermal expansion difference between the metal base material 11 and the ZrO2-based ceramic layer increases as the turbine entrance temperature increases. This thermal stress causes peeling of the topcoat and leads to degradation of the durability of the thermal barrier coating film. Improvements are thus needed to prevent the peeling of the topcoat.
Attempts have been already made to produce a ZrO2-based ceramic of columnar crystal form by the application of an electron beam physical vapor deposition in the process of laminating the topcoat ceramic layer 13. Attempts have also been made to produce microcracks in the thickness direction of a ZrO2-based ceramic while forming the ZrO2-based ceramic by thermal spraying. According to these methods, the peeling of the topcoat can be prevented since the thermal stress caused between the base material 11 and the ceramic layer 13 is alleviated.
Also, a partially stabilized ZrO2 which is partially stabilized by addition of Dy2O3 in place of Y2O3 (hereinafter referred to as “DySZ”) is gathering attention as a ceramic material which is approximately 20% lower than YSZ in thermal conductivity.